How can I pull this inert gas asphyxiation meme off?

I purchased a strong CPAP Mask and 6ft CPAP tubing from Amazon just like it states. Never used them before but I assume it shouldn't be too difficult. How do I go about obtaining and ensuring a proper purity (not diluted by Oxygen) Helium Tank?

Also, what are the complications? Is there a strong likelihood of something going wrong?
>Suffocating partially but not until death
>Burst lung
>Discovery during the act
>Chance of people stopping you
>ect.
From what appears to be on this image, likelihood of partial suffocating seems highly unlikely if not impossible unless discovered mid-process. It also says "you will be dead shortly afterwords" - What does this mean? 1 min? 5 mins? 30 mins? 3 hours? I don't know what "shortly afterwords" means.

Medical-anons pls give advice.